Mine is pretty much always off while I'm on campus. My phone doesn't go into "power save" when I have no service, so it'll drain in an afternoon searching for a signal while I'm at school (I spend most of my time in the basement of a large concrete building, in an insulated practice room).
Otherwise, I turn it off when I don't want to deal with people, but want them to realize I'm not ignoring them - I just don't want to deal with them.